The invention relates to the control of fluid flow in oil wells.
An oil well is drilled using a drill attached to drill pipes and, after drilling, casings of successively decreasing diameters are inserted into the drilled hole, with the final casing, the production casing, conveying the oil from the well to the well head.
Various fluids are pumped down both the drill pipes and the casing string--collectively referred to as "tubing" or "tubes"--and there is a need to control the flow of such fluids. For example, the succession of casings are cemented in position to, for example, prevent drilling fluid from circulating outside the casing and causing erosion. Cementing is also necessary in the casings close to the surface to seal off and protect fresh water formations, provide a mounting for blow-out preventer equipment and for supporting the inner casings.
Cementing is achieved by preparing a cement slurry and then pumping it down the casing. As it is pumped down, the cement slurry displaces the mud already in the casing and passes out of the end of the casing and then up the exterior of the casing, displacing the mud in front of it. When all the mud has been displaced and the cement slurry is therefore continuous around the outside of the casing, pumping stops and the cement is allowed to set. The end of the casing includes a one-way valve which, when cementing is complete, prevents the cement passing back up the casing.
The cement slurry has a density which is greater than the density of the mud which it displaces. This can result in the phenomenon of `U tubing" in which the forces resisting the flow of cement are insufficient to allow the pumping pressure to be maintained and the cement slurry falls in the casing under the effect of gravity faster than the pumping rate. Accordingly, when `U` tubing occurs, the cement slurry is no longer under the control of the pump.
This is undesirable because the increased flow rates in `U` tubing can cause a strongly turbulent flow which can erode seriously any weak formations around the casing and cause laminar flow, an undesirable flow regime while equilibrium is being sought. Further, it can result in a vacuum being formed behind the `U` tubing cement slurry and the slurry may then halt while the pump slurry fills the vacuum. It can also cause surging in the rate at which the mud is forced to the surface and this can be difficult to control at surface without causing unfavourable pressure increases downhole.
In addition, during drilling of the oil well, drilling mud is pumped down the drill pipe to remove drilled material to the surface. If the drill pipe develops a leak, the volume of fluid at the drill bit is reduced and this can have adverse consequences. The drilling mud may eventually break the drill pipe at the leak. It is therefore necessary, when this occurs, to remove the whole drill pipe and examine each section in turn. This examination can be very time consuming in a drill pipe which is many thousands of metres in length.
It can also be necessary to pump successively through the drill pipe two or more fluids of differing viscosities. It can be useful to know the position along the drill pipe of the "front" between successive fluids.